Combined cycle power plants are known for their high efficiency and relative flexibility in meeting peak load and base load power demands. The combined cycle power plant typically includes at least one gas turbine power plant, one steam turbine power plant and a heat recovery steam generator (HRSG) which thermally couples the two individual power plants to form a combined cycle power plant. The HRSG produces steam for the steam turbine by utilizing gas turbine exhaust gases in a heat exchange relation with turbine feedwater. The HRSG is a stack structure which includes serially arranged heating sections, which include a superheater, an evaporator and a high pressure economizer. The HRSG also includes a steam drum which is connected to the aforementioned heat exchange sections.
The foregoing arrangement is made more efficient by the inclusion of a feedwater preheating cycle which is used as a means for preheating and otherwise conditioning (deaerating) turbine condensate feedwater. A flash tank is included in this feedwater preheating circuit in order to provide steam to the deaerator for maintaining deaerator pressure to heat the feedwater to a level such that dew point corrosive chemical reactions cannot damage the heat recovery steam generator and to remove noncondensibles which can cause oxidation within piping and heat recovery steam generator tubing. The deaerator is operated at a constant pressure which is maintained by a pressure control valve located upstream from the deaerator. The flash tank also drains into the deaerator to provide direct contact heating of the turbine feedwater. Excess steam from the flash tank is directed to a low pressure admission and bypass system associated with the steam turbine. The low pressure admission and bypass system provides regulation of the flash tank pressure while directing the steam flow either to the condenser through the bypass valve or to a turbine low pressure section through the low pressure admission valve. In the prior art the flash tank pressure set point is a constant established by the value of the crossover steam pressure at peak steam turbine load. Mechanical design of the steam turbine in a tandem compound arrangement makes the crossover piping a convenient steam admission point.
At rated steam conditions, steam flow is required from the flash tank to the deaerator to maintain deaerator pressure. As plant load is decreased, condensate flow decreases while the flash tank water flow remains constant in volume as well as energy content. At somewhere between 70% to 80% turbine peak load, steam flow from the flash tank to the deaerator is no longer needed. Once the steam flow control valve to the deaerator is closed, pressure control of the deaerator is lost. The deaerator pressure is now primarily determined by the energy entering with the flash tank water flow. Damage could result in the deaerator with a sudden increase in turbine condensate flow to cause an uncontrolled drop in deaerator pressure and the subsequent eruptive boiling. The invention as proposed will provide a means for relieving the excess energy problem by promoting the increased production of steam which is then input into the low pressure admission section of the steam turbine. The benefit obtained is higher efficiency in the cycle while solving the problem of excess energy during less than peak operation.